If you looked only at the blue states, you probably wouldn’t think that Donald J. Trump was in huge trouble. The recent 10-point polling deficits in states like Michigan or Pennsylvania may sting but aren’t jaw-dropping.

But the red states are a totally different story. Polls show him trailing or barely ahead in reliably Republican states like Georgia, Arizona and Utah, even Kansas. He has even trailed in some of these states when the national polls were more favorable to him than they are now.

It’s not a straightforward demographic story. Despite his struggles in some states neighboring Nevada, Mr. Trump still seems competitive in the state, perhaps the most diverse of the Obama-era battleground states.

What’s going on?

He is still expected to hang on and win most Southern states, but his strategy just doesn’t work as well in all the red states, especially in the South.

The simple way to think about Mr. Trump’s strength is in terms of education among white voters. He hopes to do much better than Mitt Romney did in 2012 among white voters without a degree so that he can make up the margin of Mr. Romney’s four-point defeat and overcome the additional losses he’s likely to absorb among well-educated voters and Hispanic voters. Even when Mr. Trump has led in the polls, he has fared worse than Mr. Romney among those two groups.

On paper, that’s easy enough in a state like Iowa or Ohio, where white working-class voters are plentiful and President Obama won a lot of them. They might now be willing to reject Hillary Clinton.

But that doesn’t work so well if there aren’t many white working-class Democrats for Mr. Trump to win over, or if there are a lot of well-educated voters for him to lose.

That’s more or less the situation in Georgia, and across many red states. According to our estimates, Mr. Obama probably didn’t even win 15 percent of white voters without a degree in Georgia in 2012.

So where is Mr. Trump supposed to make gains among white working-class voters in Georgia? Is he really going to drive Mrs. Clinton down to something like 5 percent of white voters without a degree? That’s not credible. His best bet is to increase turnout, but that’s less powerful than persuasion: Flipping a voter gives you one more vote and takes a vote away from your opponent; increasing turnout gives you only one more.

At the same time, white voters represent a smaller share of the electorate in the Deep South. In Georgia, just 58 percent of registered voters are white. Put it together, and white working-class Democrats make up just a sliver of the electorate. Even if Mr. Trump could make big gains with them, they wouldn’t move the needle very much over all.

In comparison, there are a lot of well-educated Romney voters in a state like Georgia; Mr. Romney won around 75 percent of white voters with a degree. So there are far more college-educated white voters for Mr. Trump to lose than there are white working-class Obama voters for him to flip.

This basic formulation — the number of white college-educated and Hispanic Romney voters compared with the number of white working-class Obama voters — goes a long way toward explaining many of the shifts in the 2016 electoral map.

Mr. Trump is losing ground in most of the places where there are far more college-educated white Romney voters. The Clinton campaign has stopped airing advertisements in Virginia and Colorado. Poll after poll shows Mrs. Clinton competitive or even ahead in states like Georgia and Arizona, where Mr. Obama was not competitive four years ago.

Yet Mr. Trump is holding up pretty well in the places where there aren’t very many well-educated white voters or Hispanic voters to lose, even in diverse areas where one would expect Mr. Trump to struggle, like Nevada. Many of the states where Mr. Obama was competitive in 2008 but where Mrs. Clinton is not especially competitive, like Montana, Indiana and Missouri, fall into this category.

Iowa is one of the most extreme cases, and it’s also one of the few states where polls suggest Mrs. Clinton is faring worse than Mr. Obama did four years ago.

In practice, Mr. Trump’s strength and weakness might not align perfectly with this map. For instance, he could prove to be especially weak among college-educated voters in one state but more resilient in another.

Mrs. Clinton seems to be showing that kind of uneven appeal with working-class whites. According to a huge compilation of SurveyMonkey polls totaling nearly 90,000 respondents, Mrs. Clinton is facing big losses among white voters without a degree in the Northeast and Midwest, but barely losing any ground with them in the South and West. Meanwhile, she appears to be making big gains among college-educated voters in just about every region, including the South.

The shifts among white working-class voters largely mirror those from the Republican primary, when Mr. Trump excelled among white voters across the eastern half of the country. But in the general election, it does not appear that there’s much more room for Mr. Trump to gain among white working-class voters in the South.

With that kind of a ceiling among those voters in the nation’s most reliably Republican region, there seems to be room for him only to fall.